Optical wavelength division multiplex systems (WDM) use multiple laser beams on different wavelengths to transmit simultaneously multiple data channels over a common optical fiber. On the receiving side the channels are separated by wavelength filters. The narrower the filter pass-bands, the more channels can be placed. For correct operation, the laser wavelength of a particular transmitter must fit to the pass band of the wavelength selector at the receiver side.
The requirement, that laser wavelength and filter pass-band must not diverge from each other is challenging with decreased filter bandwidth. Nevertheless, in existing systems, there is no means to actively control the wavelength fit between laser and filter. Conversely, lasers are offline tuned to the absolute wavelength values as defined by the ITU wavelength grid. The laser wavelength is tuned offline by temperature under control of an optical spectrum analyzer. Later in the field, i.e. during normal operation, the established temperature set point is kept constant.
Optical burst mode by its nature cannot guarantee constant optical power in the laser. Dense traffic yields high average power, sparse traffic results in low average power. The traffic density can change at any time scale, ranging from milliseconds to hours or years. Most optical packet sources react to power changes with small wavelength deviations. The filter pass-bands must thus be made large enough to absorb the remaining tolerances. Moreover, this strategy requires a high degree of long term stability of the laser device itself as well as its driving circuitry with respect to ageing and environmental changes.
Semiconductor laser devices are in principle tunable. However, this inherent tunability of a laser cannot be used today, because there is no simple feedback available regarding the actual wavelength. The indirect control by temperature and laser current is technically usable only for a fix set point. Even the tunability of laser power (required in TDMA burst mode) is critical, as the laser power influences the wavelength at constant temperature.
In general, WDM lasers are temperature controlled by a thermo-electrical cooler (TEC) under control of a thermistor. However, between the active zone of a laser chip and the basement remains a small temperature difference, which is outside of the control loop. Even with ideal temperature control of the basement, this temperature difference (and thus the wavelength) of the active laser zone is power dependent. If power changes, the wavelength will change too, certainly with some delay. FIG. 3 shows the measured thermal wavelength transient of a temperature stabilized DFB laser when switched between zero and nominal power. The transient starts within the first micro second, but it settles only after tens of milliseconds (logarithmic time scale!). In translation to the above mentioned burst mode traffic dependence, the left side of FIG. 3 corresponds to a sparse flow of short packets (<1 μs). The right side corresponds to a 100% occupied channel. Transitions between both operating conditions can take place at any time and last for undetermined period. State-of-the-art temperature controllers are by 6 or 7 decades to slow to compensate this effect. Nevertheless, the wavelength shift (0.5 nm) is, even if not compensated, still in range of a 200 GHz ITU wavelength grid with 1.6 nm channel spacing.
It is an object of the present invention to improve the wavelength control of a laser in a WDM system.